Diaphragm and Posterior-Wall Muscles
- Posterior-wall muscles: The diaphragm forms the superior boundary of the abdomen. Psoas major flexes the hip and trunk, quadratus lumborum laterally flexes the trunk and fixes the twelfth rib, and iliacus flexes the hip.
- Psoas major lies beside the lumbar vertebrae, iliacus fills the iliac fossa, and quadratus lumborum spans between the iliac crest, twelfth rib, and lumbar transverse processes.
- Psoas fascia: It contributes to iliac fascia and can transmit infection from the posterior abdominal wall toward the thigh. A psoas abscess may therefore track inferiorly.
- A fascial plane is a connective-tissue pathway that can guide the spread of fluid, pus, or infection between body regions.
- Posterior-wall innervation: The subcostal nerve and branches of the lumbar plexus supply posterior-wall muscles. Their courses follow the fascial planes of the posterior abdominal wall.
- Diaphragmatic openings: The inferior vena cava passes at T8, the esophagus and vagal trunks at T10, and the aorta, thoracic duct, and azygos vein at T12. These levels provide important surgical and imaging landmarks.
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